We have focused attention on the 1.2 kb insulator DNA sequence at the 5 end of the chicken beta-globin locus, and elements upstream of it. This insulator is capable both of blocking the influence of outside enhancers and of preventing the encroachment of condensed chromatin that might shut down expression of the entire region. We have shown previously that enhancer blocking activity is associated with binding of a single protein, CTCF, to a site within the enhancer. We have shown that this protein is responsible for regulation of imprinted gene expression at several imprinted loci. In order to understand the mechanism of action of CTCF we have determined the minimum CTCF fragment that can bind to its target DNA site with full affinity. We have also extended our earlier studies showing that CTCF molecules interact with one another. This provides important information for design of proteins that can modulate CTCF function within the nucleus. In earlier studies we also showed that CTCF interacts with the nucleolar protein nucleophosmin. We are now extending this to determine what other proteins interact with CTCF and could modulate its function, particularly in stabilizing long range interactions in the nucleu